Infra-red intensities in CH2F2, CH2Cl2 and CF2Cl2

The integrated intensities of the fundamental infra-red absorption bands of CH₂F₂, CH₂Cl₂ and CF₂Cl₂ have been measured using the pressure-broadening technique.

Experimental results are interpreted in terms of displacement polar tensors, which give the change of the dipole moment of the molecule due to a set of displacements of the nuclei. The components of these polar tensors have been calculated for the nuclei of C, H, F and Cl for each of the studied molecules. The results obtained show that the bond moments hypothesis, used by most authors in the interpretation of infra-red intensities, is only a rough approximation. Thus, we have found that a bond stretching gives rise to a dipole moment change not directed along the bond but strongly deflected towards the most polarizable atom of the molecule. Similar results have been obtained for the bond bendings.